This invention relates to systems and methods for processing electrically floating substrates, either single sided or two-sided, using plasmas created through generated ions and, more particularly, to processing systems and methods for controlled treatment of substrate surfaces.
Commercial plasma sources are used for both controlled deposition onto and etching from surfaces for a wide variety of industrial applications, especially semiconductor, optical, and magnetic thin film processing. The plasma formed by such sources generates reactive neutral and ionic species which can chemically and/or physically interact with surfaces to deposit or remove material.
In many processes, the use of energetic ions from plasma sources can result in the deposition of materials with unique properties or allow the etching of surfaces under conditions which would not otherwise be effective. A method for processing substrates in a plasma generally includes an ion source mounted in a vacuum chamber in which the substrate is present. A gas with specific chemical properties is supplied to the ion source for ionization. The plasma generated is a mixture of selected reactive neutral and ionic chemical species as well as energetic electrons. The energy of the ionic species interacting with the surface depends upon plasma electrical properties, the electrical potential of the substrate and chamber pressure. In the prior art, the energy of ions bombarding the substrate is controlled by means of the bias applied to the substrate. In the present work there is disclosed an alternative wherein the substrate is electrically floating and acquires a net charge thereby establishing the potential of the substrate. The ion energy is determined by the difference between the plasma potential and the potential at the surface of the substrate for which there is zero net current. The floating potential of the substrate is controlled in accord with the present invention.
For a wide variety of plasma based processes a critical parameter for the treatment of a substrate is the kinetic energy of the ion(s) intercepting the substrate. The ion kinetic energy is a probabilistic function of several variables characterizing the plasma, such as the pressure, temperature, the specific plasma gas, ion source parameters and the like. The potential of the substrate is a major contributing variable to the kinetic energy. For the case of a conducting substrate, this potential may be controlled by direct connection to an appropriate power source, as commonly practiced in the prior art. In the extreme case of a dielectric substrate, such a procedure can not produce a uniform constant potential over the surface of the substrate. As described herein, the present invention is directed to any situation wherein direct coupling to a power source will not suffice to control the substrate potential or such electrical coupling to the substrate is otherwise undesirable. The present invention is not limited to a perfect dielectric substrate, nor is it limited to the specific processes which are disclosed herein as exemplary exploitation of the invention
In some applications, it is desirable to process both sides of a substrate simultaneously. This is typical in the deposition of thin layers of various materials in the manufacture of magnetic hard disks used in magnetic memory systems. In this case, ion sources are positioned on opposite sides of the disk. However, ion sources which utilize an anode for establishing a plasma potential tend to exhibit plasma instability and oscillation when two such sources are operated simultaneously in a processing chamber. Such unstable behavior does not permit predictable ion generation and process stability. Prior co-pending application Ser. No. 076,971 addressed this problem by a time division multiplex of depositions by the respective ion sources thereby obtaining symmetrical coatings of the respective surfaces of the substrate. Also, it has proven difficult to coat thin films to the specifications satisfying the requirements of a protective film on a hard disk, for example, for computer data storage applications. Thinner coatings permit the head to fly closer to the magnetic domains at the surface of the disk as to permit an increase in Arial density of recorded information. In depositing overcoatings of the magnetic surface, the coating should have sufficient hardness, density, and adhesion as well as practical qualities in the finished disk, including high deposition rates and low numbers of resulting macroscopic particles on the surface. Accordingly, there is a need for improved substrate processing systems and methods wherein ion sources may operate in a stable manner in a processing chamber and wherein the properties of the deposited layers may be improved for their intended purpose.
Co-pending applications referenced above, taught the advantage accruing from differential biasing of substrate and chamber walls whereby the deposits on the chamber walls were characterized by low internal stresses resulting from a lower ion energy whereas the thin film material concurrently deposited on the substrate possessed desirable characteristics of hardness, density and adhesion resulting from deposits from ions of higher kinetic energy relative to the substrate.
These same practical requirements noted above are appropriate to optical as well as magnetic media. For example if a protective coating is desired for an optical substrate, uses of the disk require that coatings that are deposited be deposited with the desired hardness, density and adhesion at a high rate while extremely thin and that variations through the presence of varying particles be minimized.
According to a first aspect of the invention, a novel substrate processing system is provided. The substrate processing system comprises a processing chamber, an electrically floating substrate holder positioned in the chamber, a gas source for supplying a process gas to the processing chamber, at least one ion source located in the processing chamber and a power source for applying various voltages to the ion source or sources (in the event more than one source is present), and to also energize other surfaces of the chamber and a controller for regulating the duty cycle of the time dependent electron source portion of each ion source. Each ion source ionizes the process gas to produce ions for processing a substrate disposed on the substrate holder. Each ion source has a cathode and an anode. Each ion source also produces sufficient electron flux of appropriate energy distribution to produce a net negative charge accumulation on the substrate in the presence of an active plasma, to further lower the substrate potential. The power source energizes the one or more cathodes of the ion source or sources as well as the one or more anodes. In the event that more than one ion source is being used, the power source energizes the ion sources in a time multiplexed manner such that only one of the ion sources is energized at any time.
The controller senses chamber pressure through a pressure sensor and also monitors such electrical parameters as electron source emission current and anode and cathode potentials (of each ion source). By controlling these parameters, a desired substrate potential can be maintained.
The energy and density of electrons emitted by the cathode determine the net charge accumulation on the substrate, thereby controlling the substrate potential. The energy spectra of the electrons emitted by the cathode is controlled by the voltage difference between the anode and cathode, while the density of electrons emitted by the cathode is determined by the emission current (rate of electrons leaving the cathode) and the transport of electrons to the wall. In order to obtain a significant range of substrate potentials, some form of electron confinement is required, either with the use of magnetic fields (such as multipole fields) or electrostatic fields (cathode potential equal to or greater than wall potential). A different embodiment of the invention could use RF waves to produce the plasma and heat the electrons. The present invention employs a floating substrate and utilizes values of cathode and anode potentials and low enough gas pressure to assure that a portion of the electrons emitted from the electron source portion of the ion source will have sufficient kinetic energy the negative charge accumulation on the substrate causes the substrate potential to acquire a desired negative potential.
Biasing of elements of an ion source and/or the effective substrate potential as achieved herein, may also be used in accordance with this invention to selectively control the energy of the ions from the ion source that reach and interact with particular surfaces. For example, adjusting the potential of the substrate or by biasing the elements of the ion source one can concurrently create higher energy ions for deposition at the substrate and lower the energy of ions which deposit on the chamber walls. Thus, this invention enables a predetermined control of the condition of deposition at a surface and permits selectivity as to properties of the film deposited in accordance with ion energies.
In particular, when forming thin films of diamond like carbon (DLC) from a plasma containing a hydrocarbon gas such as ethylene, carbon deposited with low energy ions ( less than 100 eV) on the walls of the processing chamber will exhibit much lower stress than carbon formed on the substrate with more energetic ions (180-200 eV). As the carbon deposits on the wall builds up, the lower stress will make it less likely to flake, and lower levels of small particles will be present on the surface of the substrate being coated. Although in this example the layer deposited on the substrate may exhibit higher stress then the deposited carbon on the wall, this is not a problem for the system because the deposited layer is so thin and the hard deposited surface acts as a most effective protective coating for the layers below.
Where the substrate is an insulating material, the prior art employed the selectable energy of an ion beam. See Kimock, et al, Datatech, 2nd Edition, Spring 1999 Edition, pp. 69-77 (Published by ICG Publishing Ltd., 14 Greville Street, London EC1N 8SB). Typical prior art deposition apparatus employs biased grid structures to accelerate the ions. Such grid structures are essentially formed of a conductor defining an array of apertures through which ions accelerate from the plasma. Such apertures are limited in dimension to a few Debye lengths (a measure of the length to which an electric field extends into a plasma), thereby limiting the plasma density. The ion current produced is space charge limited. As a result, the deposition rate is rather low and throughput for the process is correspondingly low. It is also well known for the plasma at high potential to be isolated by spatial localization with magnetic fields. Such an arrangement requires a significant increase in the dimensions of the processing chamber to accommodate a transition region and a low background neutral pressure to avoid ion collisions over this transition region.
The present invention employs an electron flux to charge the electrically floating substrate (whether or not a dielectric material) to control the potential of the substrate with respect to operative potentials of the surrounding plasma, chamber walls and the like. The electron source for this non-contact biasing is modulated to produce the desired substrate potential during deposition (or other process) pulses.
Although the electron source may take different forms, a hollow cathode embodiment is preferred to provide both plasma excitation and substrate biasing during respective intervals of an operating cycle. The stability and control of the hollow cathode source is effected in regulation of the cathode duty cycle for a relatively high frequency modulation of the hollow cathode source relative to lower frequency modulation of anode pulses where plural plasma sources are employed. This modulation is applied at a high frequency relative to the anode pulse to produce a cathode duty cycle variation on the electron current output from the cathode. It is to be understood that reference to xe2x80x9ccathode duty cyclexe2x80x9d throughout this work means the fraction of time that the cathode is biased xe2x80x9cONxe2x80x9d, allowing electrons to be emitted by the cathode, while any anode is xe2x80x9cONxe2x80x9d.
The present invention recognizes that the potential of the substrate is directly affected by the net accumulation of electrical charge thereon. In a preferred embodiment for the present application, it is desired to achieve and maintain a preselected negative potential for the substrate relative to ground (the chamber walls) but intermediate such grounded walls and the more negative potential of the ion source cathode. This is obtained in the present invention, in part, with a judicious choice of the maximum kinetic energy of the electron flux. The electrons emitted from the cathode (of a typical source) exhibit a relatively broad spectrum as schematically illustrated in FIG. 2, showing the high energy tail portion of the electron energy distribution produced by the cathode electron source. Electrons having kinetic energy less than a threshold Vthresh=e(Vplasmaxe2x88x92Vfloating), where e=the charge on the electron, will be repelled by the substrate surface. The quantity Vthresh is principally a function of the nature of the plasma and the gas pressure. Electrons of kinetic energy greater than the threshold transfer some kinetic energy to potential energy, but these electrons have sufficient energy to reach and intercept the substrate. Thus, these higher energy electrons contribute to charge the substrate to net negative potential in equilibrium with the positive ions bombarding the substrate from the plasma so that there is net zero electrical current to the substrate. The shape of the distribution and the value of the threshold determine the equilibrium potential of the substrate. Regulation of the deposition rate is achieved by controlling the time average number of ions which reach the substrate by controlling the time average electron emission current.
One desideratum of the present invention is an efficient and controllable electron flux. A preferred embodiment is one wherein the electron flux is spatially uniform in a desired direction and the predominant direction of electron emission from the cathode is directed toward the substrate rather than emission into 4xcfx80 steradian and wherein the electron energy distribution is characterized by a reliably known shape providing a reasonable fraction of high energy electrons. The hollow cathode ion source answers these requirements.
The plasma density is enhanced by placement of a number of discrete permanent magnets along the chamber walls with the magnetic axis transverse to the axis defined by the centroid of the cathode and the center of the substrate. The adjacent magnets are thus disposed azimuthally about the chamber with alternating polarity to produce a greater and more uniform plasma density due to the resulting multipole magnetic field. These magnetic fields also enhance the confinement of the energetic electrons, thereby increasing the density of energetic electrons, which increases the difference in potential between the plasma and substrate.
Thus, prescribed plasma based processes which depend upon a controlled substrate potential are implemented in a novel manner. In particular, thin layers of DLC coatings can be deposited as protective coatings on one or both sides of magnetic, dielectric or other media (optical for example) with superior results compared to sputtered carbon films. In this instance the deposited layers, deposit as hard protective layers with sufficiently low numbers of small particles to minimize disk rejections in the manufacturing process resulting from glide or certification yield tests typically used by manufacturers to determine whether satisfactory and merchantable disks have been made. In the event that a disk fails to pass such tests the manufacturer may be obliged to discard the disks reducing output and profitability. The current invention considerably reduces these problems.